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AP Chemistry

Name: ___________________________ Group Members: ___________________________ ___________________________ ___________________________

Flame Test

Purpose and Objectives

If an element can be placed in solution and that solution aspirated into a burning flame, the element’s electrons will absorb energy. This process is sometimes called “exciting” the electrons. As those electrons then return to their normal or “ground” state, the energy absorbed must be emitted in the form of electromagnetic radiation. Every element emits a characteristic wavelength of light energy during this process. If the wavelength of electromagnetic radiation emitted falls within the range of visible light, it can be seen as a characteristic color of light energy. Just as a fingerprint is unique to each person, the color of light emitted after excitation of an element is unique to that element. Only a few elements emit a characteristic light within the visible region of the spectrum. For most elements the characteristic wavelength falls within the ultraviolet (UV) or infrared (IR) region of the spectrum. Barium, calcium, lithium, potassium, copper, boron, sodium, and strontium are examples of elements that display a characteristic light in the visible region of the spectrum. Salts of these ions will be used in this demonstration. This lab utilizes q-tips or wooden splints dipped with a solution of a variety of salts to produce a brilliantly colored flame. When the solution is placed into the flame of a burner, a spectacular ball of colored fire forms that is easily visible in even large classrooms.

Equipment

CaCl 2 , NaCl, KCl, LiCl, BaCl 2 , CuCl, (or substituted compounds) wooden splints, distilled water

Procedure

AP Chemistry

1.

Repeat with each solution. 2.

Prepare a beaker of water. 3.

Obtain the vials with salt solutions. Lower the wooden splint into one of the samples. (If 4.

using wooden splints, you will need to allow them to soak overnight.) Light your Bunsen burner. 5.

Lower the q-tip into one of the samples – (If using wooden splints, remove from solution). 6.

Place it in the flame and observe the color. Allow the splint to burn until the color disappears. Try not to get the salts on the Bunsen burner. Immediately place the burned end of the splint into the beaker with tap water to put it out. 7.

Record the color of the flame in the data table below. 8.

Repeat steps 5 - 7 for the remaining salts. 9.

Clean up: place the used wooden splints in the solid waste container. Tightly screw the caps on the salt samples and put them back in the designated bin. Rinse and dry the beakers you used. Wash your hands.

AP Chemistry

Name: ___________________________ Group Members: ___________________________ ___________________________ ___________________________ FLAME TEST LAB DATA SHEET

Data Table

Metal Ion

Lithium Sodium Potassium Calcium Barium Copper (I)

Color of Light

Violet Blue Blue-green Green

Symbol w/ Charge Color of Flame Wavelength (nm) Wavelength of Light (nm)

410. 470. 490. 520.

Color of Light

Yellow-green Yellow Orange Red

Wavelength (m) Wavelength of Light (nm)

565. 580. 600. 650.

Analysis Questions

AP Chemistry

(Show all work – don’t forget units!) 1.

2.

3.

4.

5.

6.

7.

What is the relationship between frequency and wavelength? For Li + , Na + , and K + : a.

b.

c.

calculate the frequency of emitted light calculate the energy emitted with one photon in Joules calculate the energy released when one mole of electrons returns to the ground state in kJ mol –1 . Rank the Group 1A elements tested in order of increasing energy emission when 1 mole of electrons returns to the ground state. Explain this pattern in terms of changes in energy levels. An excited electron falls from n = 4 to n = 2. a.

Calculate the energy change in Joules associated with this transition. b.

c.

What is the wavelength in nm of the emitted electromagnetic radiation? What color is it? Calculate the energy change in kJ mol –1 . An excited electron falls from n = 3 to n = 1. a.

Calculate the change in energy in Joules associated with this transition. b.

c.

b.

What is the wavelength in nm of the emitted light? How is this different from the electromagnetic radiation emitted in Question 5b? Could you detect this light with your eyes? Calculate the energy change in kJ when one mole of electrons moves from the third principal energy level to the first principal energy level. A certain line in the spectrum of Hydrogen is associated with the electronic transition from the fifth energy (n = 5) level to the third energy level (n = 3). a.

In the electronic transition described above, does the atom emit or absorb energy? Justify your answer. Calculate the wavelength of the photon associated with this spectral line. Account for the observation that the energy associated with the same electronic transition in the He greater than that associated with the corresponding transition in the H atom. + ion is When a chloride solution of an element is vaporized in a flame, the color of the flame is violet. What element could be in the solution?

Conclusion

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